Recently, an image pickup camera for broadcast TV, e.g., a so-called E cinema camera capable of recording an image for presentation on a cinema screen, has been developed. A wide-angle imaging lens used in an E cinema camera requires a wider angle and higher performance over the entire image plane than a common image pickup camera for broadcast TV.
Moreover, when a fixed solid-state image pickup element is used in such an E cinema camera, similar to its use in a common image pickup camera for broadcast TV, a low-pass filter and an infrared rejecting filter may also be provided between the imaging lens and the solid-state image pickup element. In that case, room for those filters must be provided in the back focus region of the imaging lens. Furthermore, when imaging in color with an image pickup element for each primary color, a color separation prism is provided in front of the solid-state image pickup elements, and providing room for that prism requires an even longer back focus of the imaging lens. Additionally, internal focusing is preferably used to reduce variations in aberrations and field angle usually associated with focusing in an image pickup camera for broadcast TV.
Japanese Laid-Open Patent Application H04-118612 discloses a retrofocus wide-angle lens that provides very good performance even when the field angle is 90xc2x0 or more. However, the front lens group of that wide-angle lens is too large for the desired compactness of the image pickup camera. For example, in the retrofocus wide-angle lens of Embodiment 1 of that application, with a focal length of 5.0 mm, an FNO of 1.8, and field angle of 95.5xc2x0 (for an image circle of 11.0 mm), the optically effective diameter of the first lens group is 95 mm. However, there is a demand for making it about half that size.
Additionally, the retrofocus wide-angle lens disclosed in Japanese Laid-Open Patent Application H04-118612 is constructed so that focusing is performed by moving lens components in both front and rear lens groups, which requires making complicated mechanisms for focusing. Furthermore, focusing causes significant variations in field angle, which is undesirable in E cinema cameras.
As mentioned above, internal focusing is used in order to reduce variations in aberrations and field angle with focusing. Besides the retrofocus wide-angle lens disclosed in Japanese Laid-Open Patent Application H04-118612, Japanese Laid-Open Patent Application 2000-131606 discloses a similar retrofocus wide-angle lens that uses internal focusing with a positive lens group and a negative lens group that are included within a first lens group. Focusing is achieved by moving the positive lens group that is on the image side of the negative lens group. However, for the retrofocus wide-angle lens described, the ratio of back focus to the focal length of the entire retrofocus wide-angle lens (i.e., the retrofocus ratio) is about 2.6, which is not always sufficient.
In addition, with retrofocus wide-angle lenses as described above, correction of chromatic aberration is very important. In responding to this requirement, the use of anomalous dispersion glass in a positive lens element of the second lens group is known. The term xe2x80x9canomalous dispersion glassxe2x80x9d is used herein to mean glass having a large deviation in the partial dispersion ratio xcex94 xcex8 g, F (i.e., a large xe2x80x98anomalous dispersionxe2x80x99) from a line connecting normal glasses F2 and K7 when plotted on a graph with the Y-axis being the partial dispersion ratio (xcex8 g, F) and the X-axis being the Abbe number xcexdd. For a complete explanation of what the xe2x80x98partial dispersion ratioxe2x80x99 is and how it is obtained, please see U.S. Pat. No. 5,858,898 (especially FIG. 1 and the paragraph spanning columns 6 and 7), the contents of which are herein incorporated by reference.
The present invention relates to a high performance retrofocus wide-angle lens, suitable for TV-type cameras, and particularly relates to a high performance retrofocus wide-angle lens with a long back focus and with a relatively small, optically effective diameter of the lens group at the object side of the retrofocus wide-angle lens that assists in obtaining the high compactness desired in that type of camera. Further, the present invention relates to such a high performance retrofocus wide-angle lens that reduces variations in aberrations and field angle with focusing and allows internal focusing with simple mechanisms. Additionally, the present invention relates to such a high performance retrofocus wide-angle lens that corrects chromatic aberrations without allowing corresponding large changes in the back focal length that are caused by changes in temperature in retrofocus lenses in prior art cameras. The present invention is especially useful for broadcast TV cameras such as E cinema cameras.